1.1.1 The Eye

Because of the remoteness of the scale of light waves from that of
easilyobserved
phenomena, understanding the human perception
of light is not straightforward. We begin our path to understanding
by examining the way in which light is detected in the human body.
The purpose of the focussing is to cause the light to fall onto the
retina, which contains two principal kinds of photoreceptive cells:
rods and cones. These cells are sensitive to particular kinds of light,
firing under some circumstances and not others.
There are two areas of particular interest on at the back of the eye:
one is the fovea, a pit with the highest density of cones on the retina;
it is responsible for our sharp central vision, used whenever fine
detail is required. The other is the optic disc, where the nerve cells
joined to the photoreceptors congregate before going into the optic
nerve towards the brain. It is this area which gives rise to the blind
spot in human visual perception; there are no useful photoreceptive
cells in this area, as all the nerve cells come over the front of the
retina, obscuring the light.